On Sat, Sep 14, 2024 at 11:03 PM, @Ludwig_DH8WN wrote:
On Sat, Sep 14, 2024 at 04:26 AM, James wrote:
The Gate and the source voltage remains the same at 8.0V during all three "No Key", "P", "T".
If I read the tester display right, Q507 seems to be ok.
To be clear, it looks like Q507 is ok but I'm not sure because
But why is the voltage at G the same as at S?
So Q507 maybe not ok or something around Q507 is wrong. If voltage at G and at S are the same, across a well working Q507 no current is flowing and no voltage at the PA like for your QMX.
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My suggestion:
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Please check schematics and trace from gate of Q507 to R506 and collector of Q514. Check voltage of both sides of R506 and voltage at base of Q513 (signal DAC from the controller). Check also R507 for the right value and no short.
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Normally a current should flow across R506 in each situation. When "no key" and "P" depressed this current is low and the gate voltage is a bit below the source voltage but not the same. When "T" depressed the signal DAC from the controller (B of Q513) is going from nearly 0 V to more than 1 V. This results in an increase of the current across R506 and the gate voltage of Q507 drops more. This lowers the source drain resistance of Q507 and a current is flowing to T501 and the BS170. This should be.
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So you have to find why not for your QMX.
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73 Ludwig
